1. Field of the Invention
The present invention relates to a short-circuit distance relay for protecting a single-channel power transmission system with power supplies installed at two ends thereof, and more particularly to an improved distance relay which is capable of eliminating a distance measurement error induced by a voltage drop component flowing through a fault-point resistance from the remote-end power supply.
2. Description of the Prior Art
In the conventional short-circuit distance relays of this type known heretofore, there have been existent some technical problems as described in "Handbook of Protective Relays" (Ohm CO., Ltd., 2nd edition, 2nd printing, April 25, 1970, pp. 111-112); "Protective Relay Technology" (Denki Shoin, 1st edition, 1st printing, Oct. 15, 1972, pp. 323-324); "Protective Relays" (A. R. van C. Warrington, John Wiley & Sons, New York, 1st edition, 1969, pp. 270-274); and "Applied Protective Relaying" (Westinghouse Electric Corp., Copyright 1976, pp. 10-34, 10-42, 10-43 & 10-44). In the present stage of development, however, such problems have not been solved yet. An exemplary conventional apparatus will now be described below.
FIG. 1 is a block diagram of a single-channel power transmission system with power supplies installed at two ends thereof, wherein there are shown a local-end power supply 1, a voltage transformer 2, a bus 3, a voltage transformer 4, a current transformer 5, a short-circuit distance relay 6, a power transmission line 7, a fault point 8, a bus 9, a voltage transformer 10, and a remote-end power supply 11.
FIG. 2 is an equivalent circuit diagram representing occurrence of a two-line short-circuit fault at the point 8 in FIG. 1. In the figure, there are shown a fault-phase supply voltage E.DELTA.P at the local end (P), a fault-phase supply voltage E.DELTA.Q at the remote end (Q), an impedance ZgP behind the local end, an impedance ZgQ behind the remote end, an impedance Z per unit length (km) of the power transmission line, a distance X (km) from the short-circuit distance relay to the fault point, an entire length L of the power transmission line, a fault-point resistance RF, a voltage VR at the location of the short-circuit distance relay, a fault current IP flowing through the fault point from the local-end power supply, and a fault current IQ flowing through the fault point from the remote-end power supply.
In FIG. 2, the voltage VR at the location of the short-circuit distance relay 6 is given by Eq. (1) below. EQU VR=XZ.multidot.IP+RF.multidot.(IP+IQ) (1)
On the basis of the current IP flowing in the short-circuit distance relay 6, the impedance measured by the relay 6 is obtained through division of the voltage VR, which is applied to the relay 6, by the current IP as follows. EQU VR/IP=XZ+RF.multidot.(1+IQ/IP) (2)
When the fault-point resistance RF is zero in Eq. (2), the impedance to be measured is obtainable as XZ in which Z represents the impedance per unit length of the transmission line 7, so that it is possible to measure the distance from the location of the short-circuit distance relay 6 to the fault point 8. Also in case the fault-point resistance RF has a finite value instead of being zero, the above distance is measurable from the sinusoidal component of the voltage VR relative to the fault current IP if an in-phase relationship is existent without any phase difference between the fault current IP from the local-end power supply 1 and the fault current IQ from the remote-end power supply despite a variation of the second-term resistance component in Eq. (2). This is obvious as the reactance component of XZ remains unchanged.
FIG. 3 graphically shows an exemplary case with occurrence of a fault in a conventional short-circuit distance relay 6, wherein R-jx coordinates represent a vector obtained when the fault current IQ from the remote terminal has a phase lag in comparison with the fault current IP at the local end.
In the graph of FIG. 3, there are plotted a voltage drop XZIP (segment OB) up to the fault point 8 (in FIG. 1) on the power transmission line 7 (in FIG. 1); a voltage drop RFIP (segment BC) caused across the fault-point resistance RF by the local-end current; a voltage drop RFIQ (segment CD) caused across the fault-point resistance RF by the remote-end current; an impedance angle .alpha. of the power transmission line; a phase difference angle .theta. between the voltage VR and the current IP at the relay location; and a phase difference angle .delta. between the local-end current IP and the remote-end current IQ.
In the conventional short-circuit distance relay having the above-mentioned constitution, the voltage VR at the relay location is represented by a segment OD of FIG. 3 from Eq. (1) when there exists a phase difference angle between the local-end current and the remote-end current, so that the reactance component obtained from the voltage VR and the local-end current IP is erroneously equal to the reactance value of a fault at a point A when the fault-point resistance RF is zero, whereby the point A varies depending on the fault-point resistance RF, the ratio between the local-end current IP and the remote-end current IQ, and also on the phase difference angle .delta. between the local-end current IP and the remote-end current IQ. Thus, it has been unavoidable heretofore that an error is induced in measuring the distance up to the fault point.